This invention relates in general to ovens for preparing food and more particularly to an oven which relies on both heated forced air and radiant energy to cook or otherwise prepare the food.
So-called convection ovens, which use a fan to force air into circulation within an oven chamber, have found widespread acceptance in the food preparation industry, particularly in retail food stores and restaurants which specialize in having cooked food available for immediate order. Typically, these ovens rely on electrical heating elements to heat the air which is circulated, and in some the electrical heating elements are arranged such that they are also exposed to the food so that radiant energy from the elements likewise cooks the food. U.S. Pat. Nos. 3,828,760 and 4,561,348 disclose such ovens. The electrical demands of these ovens are indeed substantial. They operate on elevated voltages and draw considerable current, sometimes multiphase, and thus cannot be serviced with conventional electrical circuits. They require special circuits and often special electrical service to the buildings in which they are located. Gaseous fuels heat more efficiently, and while a gas-fired oven requires a gas line, that is often more easily installed than the specialized electrical service required for electrically-heated ovens. But gas burners are rarely found within the cooking chambers of convection ovens, because the forced air makes the flame difficult to ignite and, once the flame is lit, the forced air tends to lift it away from the burners and extinguish it. Also, while one can easily control an electrical oven by connecting its heating elements across an electrical energy source at desired intervals, a gas burner is not so easily controlled.
The more sophisticated convection ovens contain rotisseries which move the food through an orbital path so that no one item under preparation remains in a specific location as it is cooked. Since the temperature of the heated convection air may vary within the oven chamber and the radiant energy may vary even more so within the chamber, the constant rotation results in more uniform cooking of the food. Many rotisseries hold the food on spits as it is cooked, and of course, the spits undergo the orbital movement as the rotisserie turns about its axis. In some rotisseries the spits rotate about their own axes as the rotisserie itself revolves about its primary axis, and this double rotation provides even more uniform cooking, particularly where the oven contains a source of radiant energy, since each revolution of the rotisserie exposes a different surface of the food on any spit to the source of radiant energy.
Double rotation creates complexities of its own. Usually it is derived from a planetary gear system embodied in the rotisserie, with the planet gears being carried by a drive wheel that holds one end of each spit, while the other end is supported on a slave wheel. The planet gears mesh with a ring gear. Any distortion of the drive wheel by a temperature differential in it affects the mesh between the planet gear and ring gear and may cause excessive wear or perhaps binding. Moreover, distortion in the drive and slave wheels may actually separate a spit from the wheels, causing the spit to fall to the bottom of the oven chamber.
The typical rotisserie derives its rotation from a drive motor which is located outside the oven chamber and operates with conventional alternating current. As a consequence, it turns quite rapidly--indeed much faster than the 3 to 5 rev/min imparted to the drive wheel. The reduction is usually achieved in a gear box, but to achieve a reduction of that character in a single and highly compact gear box requires small gears with small teeth. Occasionally these gears fail when the planetary gears bind or in some rare cases when the rotisserie encounters a substantial imbalance.
Apart from that, spit rotation at a velocity suitable for cooking will leave a food product, such as chicken, that carries a coating or contains juices, with circumferential stripes if the rotation continues after the food product is cooked. The coatings and juices tend to remain on the surface of chicken, while flowing downwardly over the chicken, thus imparting the stripes. And many operators of these ovens hold the product in them far after they are cooked to keep them warm and to present an attractive display.
The display typically involves some type of illumination--usually quartz bulbs--within the oven chamber. Juices from the cooked food product become entrained in the forced air and coat the bulbs, rendering them less effective and causing early failure. Sometimes the bulbs break and shower glass into the food product. Moreover, the bulbs are difficult to clean.
The present invention resides in a cooking oven which utilizes forced convection air and radiation for cooking food, with the heat for elevating the temperature of the convection air and the radiant energy being derived from a gas burner. The burner has two stages so that it may be fired at two different energy levels, yet it is isolated sufficiently from the air stream to maintain a dependable and uniform flame. The invention also resides in a planetary-type rotisserie having a motor that operates at variable speeds and is isolated from the axle of the rotisserie with a chain drive or the like. The wheels of the planetary-type rotisserie have flanges which prevent distortion, even in the presence of extreme temperature differentials. Illumination for the oven chamber derives from lamps located in a light box covered with flat glass that will withstand extreme temperature differentials. The invention also consists in the parts and in the arrangements and combinations of parts hereinafter described and claimed. The cooking parameters exist within a programmable controller, and it operates the variable speed drive, the two-stage burner and sets the cooking time and temperature to meet those parameters--and once the cooking is complete operates the oven in a hold condition to keep the cooked food warm and in a condition to be served.